This invention, broadly, pertains to turbine engine indication tests by which an aviator accomplishes day-to-day or flight-to-flight monitoring of his aircraft engine. In a specific aspect the invention relates to the so-called health indication test or HIT check for gas turbine engines. More specifically the invention relates to a calculating device which facilitates the establishment of baseline conditions involved in the HIT check.
Army aviation maintenance officers are charged with keeping their turbine engines in excellent condition. However, even though diagnostic checks, test flights and "hot end" inspections are performed on a regular basis, it is important to have a simple means by which the pilot can monitor his engine. Accordingly daily engine recording concepts or engine health tests have been developed for determining the condition of the engine by relating significant performance parameters to definite standards or baseline conditions.
One such engine health test is described in U.S. Army Digest, May, 1975, on pages 12-16, by F. J. McCrory, Jr. This engine check or HIT concept monitors components exposed to the engine gas path, and it utilizes the three parameters, free or outside air temperature (OAT); exhaust gas temperature (EGT), the measurement being exhaust gas temperature in degrees centigrade; and compressor speed i.e. the revolutions per minute of the gas producer section of the turbine engine (N.sub.1), which is expressed in percent rpm. The speed of this section is regulated by controlling fuel to the combustor section.
As pointed out by McCrory the EGT will vary directly with OAT. Thus during cool weather an engine may seem healthy due to the lower free air temperature. In addition if the OAT is higher than the previous day's OAT a higher %N.sub.1 is necessary to give the same torque. In order to make a meaningful comparison, therefore, the parameters must be made independent of ambient temperatures. This is accomplished by converting N.sub.1 and EGT parameters to standard day conditions (15.degree. C., 29.92 inches Hg). Standard day conditions compensate for the varying air temperature conditions.
The engine manufacturer guarantees that a particular engine will produce a certain torque (power) with the N.sub.1 turning at a given percent and with a given EGT. However due to slight differences in manufacturing some engines are more efficient than others. Thus each engine is unique. As a result the specific N.sub.1 and EGT for any given power output (torque) will vary from engine to engine. For this reason, in addition to conversions to standard day conditions, a baseline condition for each engine must also be established.
In order to monitor engine condition and performance on a daily basis tables and EGT worksheets such as the log sheets appearing on page 14 of the McCrory reference must be prepared and placed in the engine historical record. Engine condition is based on the difference between the observed EGT and the baseline EGT. If the difference is 30.degree. or more the aircraft is grounded.
Whereas the HIT method described by McCrory in U.S. Army Aviation Digest provides the pilot with a method of checking engine health prior to flight, it is subject to errors by maintenance officers and pilots working with the tables. As McCrory points out the monitoring method is only as good as the effort and care exercised by the maintenance officer in developing the baseline EGT and the pilot in comparing his EGT with the baseline.
This invention is directed toward overcoming the possibility of inaccuracies or errors, in the present HIT method and the need for cumbersome log sheets and their extensive set up time. Rather, I have provided a quick and accurate way to establish a trend analysis of turbine engine operating conditions.